1. Field of the Invention
The invention relates to a negative differential resistance amplifier, and more particularly to a negative differential resistance amplifier suitable for a radio telescope and satellite communication which are required to operate with high frequency.
2. Description of the Related Art
As a negative differential resistance (hereinafter, referred to simply as "NDR") amplifier, one employing the Esaki diode has been known among those skilled in the art.
FIG. 1 is a circuit diagram illustrating an NDR amplifier employing the Esaki diode, reported by J. J. Tiemann, "Shot Noise in Tunnel Diode Amplifiers", Proceedings of the IRE, Vol. 48, August 1960, pp. 1418-1423.
The illustrated NDR amplifier includes the Esaki diode 91, a signal source 93, and a load resistance 95. The signal source 93 is comprised of an admittance 93A and a signal current 93B. The Esaki diode 91 and the load resistance 95 are electrically connected to the signal source 93 in cascade. Suppose that an admittance measured when viewed from an interface 3a-3b between the Esaki diode 91 and the signal source 93 to the signal source 93 is defined by (gs1+j.times.bs1), and that an admittance measured when viewed from the interface 3a-3b to the load resistance 95 is defined by (-gi1+j.times.bi1) wherein gi1 is positive (gi1&gt;0), the illustrated NDR amplifier has an available gain defined by the following equation (A). EQU Gav=gs1/(gs1-gi1) (A)
Hence, a significantly high gain can be obtained when (gs1-gi1)approaches 0 from the upper side.
In the above mentioned NDR amplifier employing the Esaki diode, the input terminal in the Esaki diode doubles as the output terminal. Accordingly, the NDR amplifier has no signal-unilaterality, and thus has a problem of, for instance, being likely to be unsteady and thus oscillate when the Esaki diodes are electrically connected in a cascade.